1. Field of the Invention
The present invention generally relates to engine decompressor arrangements that temporarily reduce compression pressure when an engine is started. More particularly, the present invention relates to such arrangements that facilitate generally synchronous decompression across multiple cylinders.
2. Description of the Related Art
Compression release mechanisms have been used in single and multiple cylinder engines to make the engines easier to start. For instance, European Published Patent Application No. EP 1 070 833 A2 describes one such mechanism. The mechanism disclosed in this publication uses a construction that opens an exhaust valve during a compression stroke.
The mechanism features a compression release shaft that extends in the axial direction of a valve system camshaft and one or more lift members that extend in the radial direction of the camshaft. The lift members selectively contact associated valve actuation devices such that the valves are lifted from the valve seats, which reduces the compression pressure developed within the combustion chamber.
The compression release shaft rotates within an axial bore formed in an end portion of the valve system camshaft. A driving unit, including a centrifugal weight and a return spring, is provided at one end of the compression release shaft and a cam for changing the position of each lift member is provided on the other end of the compression release shaft.
The centrifugal weight rotates radially outward under centrifugal force when the valve system camshaft rotates at a sufficiently high rotational speed. When the centrifugal weight swings outward, the compression release shaft, which is coupled for rotation with the centrifugal weight, rotates along its axis. The return spring of the driving unit urges the centrifugal weight inward (i.e., in a direction generally opposite to the movement caused by the centrifugal force). Thus, the return spring acts to return the centrifugal weight to its initial position and to rotate the compression release shaft in a direction opposite to that caused by the centrifugal movement of the centrifugal weight.
In other words, the compression release shaft is secured in a first position by the resilient force of the return spring until the valve system camshaft starts rotating. Once the valve system camshaft rotates at a sufficiently high speed, the centrifugal weight moves and rotates the compression release shaft to a second position.
The lift members are positioned within corresponding pin holes and can move in a radial direction of the camshaft. The pin holes are formed in such a way as to cross the through hole for the compression release shaft. A contact portion of the lift members protrudes from the camshaft at a location near the cam. The contact portion is designed to contact an exhaust valve and is formed on one end portion thereof with a weight being formed on the other end portion. The lift member extends more than halfway through the diameter of the camshaft. When the camshaft rotational speed increases, the centrifugal force applied to the weight end of the lift member increases, which ideally withdraws the lift member into the camshaft. Thus, as the rotational speed increases, the cam surface of the compression release shaft is moved to a position that no longer supports the lift members and the weighted end of the lift members reduces that degree to which the lift member protrudes from the camshaft.
In the following description, the position of the lift member where the exhaust valve is opened to reduce the compression pressure is referred to as a pressing position, and the position of the lift member where the engine is in the normal driving state is referred to as a non-pressing position. In the construction described directly above, the lift members are designed to move between positions solely by centrifugal force once the compression release shaft rotates into the position that no longer supports the lift members.